The present invention pertains to a refrigeration system to provide the cooling requirements of an olefin plant. More particularly, the invention is directed to the use of a tertiary or trinary refrigerant comprising a mixture of methane, ethylene and propylene for cooling in an ethylene plant.
Ethylene plants require refrigeration to separate out desired products from the cracking heater effluent. Typically, a propylene and an ethylene refrigerant are used. Often, particularly in systems using low pressure demethanizers where lower temperatures are required, a separate methane refrigeration system is also employed. Thus three separate refrigeration systems are required, cascading from lowest temperature to highest. Three compressor and driver systems complete with suction drums, separate exchangers, piping, etc. are required. An additional methane refrigeration compressor, either reciprocating or centrifugal, can partially offset the capital cost savings resulting from the use of low pressure demethanizers.
Mixed refrigerant systems have been well known in the industry for many decades. In these systems, multiple refrigerants are utilized in a single refrigeration system to provide refrigeration covering a wider range of temperatures, enabling one mixed refrigeration system to replace multiple pure component cascade refrigeration systems. These mixed refrigeration systems have found widespread use in base load liquid natural gas plants. The application of a binary mixed refrigeration system to ethylene plant design is disclosed in U.S. Pat. No. 5,979,177 in which the refrigerant is a mixture of methane and either ethylene or ethane. However, such a binary refrigeration system cascades against a separate propylene refrigeration system to provide the refrigeration in the temperature range of xe2x88x9240xc2x0 C. and warmer. Therefore, two separate refrigeration systems are required.
It is an object of the present invention, therefore, to provide a simplified, single refrigeration system for an olefin plant, particularly an ethylene plant having a low pressure demethanizer, utilizing a mixture of methane, ethylene and propylene as a tertiary refrigerant. This tertiary system replaces the separate propylene, ethylene and methane refrigeration systems associated with a recovery process using a low pressure demethanizer. The invention involves the separation of the tertiary refrigerant from a compressor interstage discharge and the final compressor discharge into a methane-rich vapor fraction and two levels of propylene-rich liquids so as to provide various temperatures and levels of refrigeration in various heat exchange stages while maintaining a nearly constant refrigerant composition flowing back to the compressor and with the bulk of the total return refrigerant flow going to the first stage compressor suction. This enables the tertiary refrigerant system to compete favorably on a thermodynamic basis with the use of separate compressors for separate refrigerants. This tertiary system can also be applied to an ethylene plant with a high pressure demethanizer in which case the tertiary system only supplies propylene and ethylene refrigeration temperature levels. The objects, arrangement and advantages of the refrigeration system of the present invention will be apparent from the description which follows.